Magnetostrictive vibrator



ou; 11, 1982. G. W PIERCE I 1,882,398

MAGNETOSTRIGTIVE VIBRATOR` Original Filed Aug. `1'7, 1928 Jaz ve?? o a 6803196' Mila'eiwe Patented' 0a. 11, i932 GEORGE WASHINGTON PERCE, F CAHBBIDGE, MASSACHUSETTS MAGNETOSTRICTIVE VIBRATOB Original application iled August 17, 1928, Serial No. 300,249. Divided and this application illed May 9, 1930. Serial No. 451.050.

The present invention relates to vibrators, and more particularly to magnetostrictive vibrators. The present application is a division of application Serial No. 300,249, filed August 17, 1928.

A magnetostrictive vibrator comprises a magnetostrictive 'core disposed in an electromagnetic field, such as may be established by passing an electric current through a field coil or Winding. The core may be in the form of a rod or tube, or any other desired form. Any material having suitable properties may be used for the core, but the material should obviously be characterized by comparatively large magnetostrictive effects and comparatively low vibrational decrement.

When stimulated magnetically by the field, the core becomes very slightly mechanically deformed or distorted by magnetostriction. The resulting increment of deformation may be a lengthening, or a shortening, or some other distortion, depending on the material and on the polarity of the increment of the magnetic field. This action of the magnetic field upon the core will, for brevity, be hereinafter termed stimulation. Conversely', When the vibrator is mechanically deformed or distorted, it will react magnetically upon the magnetic field by magnetostriction, with an increment of magnetization depending upon the nature of the preexisting magnetic field and the mechanical deformation, and this will produce its effect upon the electric current or voltage in the coil. This reaction Will, for brevity, be hereinafter referred to as the response, The mechanical deformation is produced by eX- citing reversible internal stresses in the core, and the core readily recovers upon the Withdrawal of the deforming forces.

If the current or voltage is alternating, the electromagnetic field created thereby Will also be alternating. The core will, therefore, increase and decrease in length, let us say, many times a second, every variation in the current producing its stimulative effect on the core., and cverydeformation of the core producing its reaction response upon the current. The core Will, in consequence, freely vibrate mechanically by magnetostriction.

Ordinari-ly, these vibrations will be quite small. When the alternating frequency is varied so as to assume a value close to, or substantially the same as, the natural frequency of mechanical vibration of the core, however, the amplitude of vibration of the core, though still small, becomes relatively quite large. The core Will then react inductively on the load to render its consumption of power critical as to frequency for frequencies near the free frequency of the core. There Will usually be more than one specific frequency of magnetization at which the core will thus resonate; for, in addition to one or more natural fundamental frequencies of mechanical vibration, it has also frequencies of vibration determined by the operation of the core in halves,rthirds, fourths, fifths, etc.

The chief object of the present invention is to provide new and improved magnetostrictive vibrators.

Other objects will be explained hereinafter and will be particularly pointed out in the appended claims, it being understood that it is intended to set forth, by suitable expression in the claims, all the novelty that the invention may possess.

The invention will be explained in greater detail in connection With the accompanying drawing, in Which Fig. 1 is a diagrammatic view of apparatus and circuits constructed and arranged to illustrate a principle of the presentinvention; and Figs. 2 and 3 are diagrammatic views of modifications.

To illustrate the principle of the invention, the magnetostrictive core is shown in Fig. 1 axially/positioned Within, and driven by, an inductive and resistive solenoid field coil 10, with clearance to permit free vibrations. The coil is provided with conductors 12 and 14 byv Which it may be connected, for simplicity, in series with a Source of alternating electromotive force, such as an alternating-current generator 16. A local battery 18 (shown in Fig. 1 in series With the source 16 and the Winding 10) applies a steady magnetizing or polarizing field to the core, over which the alternating field produced by the generator 16 is superposed. The alternating field is preferably smaller than the steady field, in order that the combined fields may not, at any time, fall to zero. The battery may be dispensed with, and the core may be magnetized electromagnetically by a local source sending a polarizing current lthrough a separate polarizin coil, or it may be permanently magnetize instead, if the vibrator has sufficient magnetic retentiveness, or the polarization may be produced by a polarlzing magnet supported near the vibrator, or the battery and a permanently magnetlzed core may be employed together. The se remarks, it will be understood, are applicable to all the various types of composite vibrators illustrated and described herein.

In order not to complicate the showing of Fig. 1, no tuning condenser or other means is illustrated therein for tuning the circuit or varying the frequency ofthe alternating current flowing therein, particularly as the core may itself be a tuned element of very low decrement, thereby dispensing with or supplementing electrical tuning of `the circuits. An important feature of the invention described in application Serial No. 158,452, filed January 3, 1927, of which the present application is a continuation in part, contemplates the use of a tuned system, as great frequency selectivity is thereby attained. The said application Serial No. 158,452 matured, on March 11, 1930, into Letters Patent No. 1,7 50,124.

The frequency of a particular mode of vibration of a rod or bar is determined by its elasticity, length and density. For somemodes of vibration, -the frequency is affected also by the width, thickness, radius, and the like, of the rod or bar. Different bodies, therefore, have different magneto- 'strictive properties. Alloys containing nickel, chromium, cobalt and steel, in proper proportions, have comparatively large magnetostrictiom Thus, if the vibrator is in the form of a rod or tube, of small diameter, the period of vibration is nearly proportional to the length of the rod or tube. A rod of nickel-steel, for example, known in the trade as stoic metal, having a diameter of one-half centimeter and a length of ten centimeters, has a fundamental period of longitudinal vibration of about 1/21,000 of a second. A rod of the same diameter ten times as long (100 centimeters2 has a period about 1/2100 of a second. ods of the same diameter and the same two respective lengths, but constituted of an alloy of iron and chromium in a particular proportion, have the fundamental periods of 1/27,000 and 1/2,700 of a second, respectively. These results are consistent with the fact that the two materials have different elasticities and densities.

As ordinary metals have their elasticityT and density slightly modifiable by changes in temperature, furthermore, such temperature changes introduce small variations in the natural period of mechanical vibration of such bodies.4 As is explained in the said Letters Patent, substantlally constant frequency, independent of the temperature, may be obtained by making the vibrator of material having a coeliicient of the ratio of elasticity to density that varies as little as possible with variation of temperature. Certain alloys of steel, nickel and chromium are known to possess substantially constant coeicients of frequency with Variations of temperature. One such alloy, constituted of 52% iron, 36% nickel and 12% chromium, is practically independent of temperature. I have found that a rod of nickel, chromium and steel has a period that is also practically independent of magnetic field strength over wide limits. A nickel-iron alloy of about 25% nickel, and another alloy of about 50% nickel have practically a zero temperature coeiicient of frequency of vibration.

The elasticity and the density determine also the Velocity of propagation of sound waves in the material. In general, the higher the velocity of the sound, the higher the frequency.- As is well known, various metals and alloys have various velocities of sound'.

Various alloys, furthermore, have difierent temperature coefficients of frequency of vibration. The values of some of these velocities and coeficients, which I have determined eXperimentally, are presented in my recently published paper on Magnetostriction oscillators,l appearing in the Proceedings of the American Academy of Arts and Sciences, vol. 63, No. 1, April, 1928.

I have discovered a number of alloys that have positive coeiicients (for which the frequency of vibration increases with increasing temperature) and a large ,number of other alloys that have negative coefficients (for which the frequency decreases with increasing temperature). One important group of alloys containing nickel and iron, with the percentage of nickel anywhere between 25% and 50% has a positive coeflicient, which varies in magnitude with the composition. A nickel-iron alloy of about 35% to 40% nickel hasthe :largest positive coefficient.

I have found that either a reduced or an enhanced temperature coefficient of frequency vibration can be obtained by the employment of a composite vibrator made up of two or more 4materials united mechanically', as by welding or soldering. A substantially zero temperature coefficient is desired for many pui'poses\ .--Composite vibrators of this nature may, indeed, be produced having frequency characteristics very different from that of one of the constituent materials alone. For example, a materialof very low sound velocity, such as lead or type metal, may be united with some magnetostrictive alloy to obtain a low frequency of longitudinal vibration with a given length of vibrator.

As another example, a material of positive temperature coefficient of frequency may be united to a material of negative coeiiicient of frequency to give a vibrator of zero tem- 1 perature coefficient of frequency. Again, a material which expands on increase-of magnetization maybe united with a material which contracts or which is neutral with increase of magnetization to produce a vibrator that operates transversely, so that when magnetized lengthwise the composite vibrator executes iiexural vibrations. Such vibrations are of very low frequency compared with the frequency of longitudinal vibrations of the body.

The vibratorA illustrated in Fig. 1 comprises a tubular body 1 of one material filled with a filler 2 of another material. At least one of these materials is magnetostrictive. The lother may or may not be magnetostrictive, depending upon the use to which it is desired to put the vibrator. Thus, the filler 2 may be constituted of lead, type metal or various alloys, if low frequency is desired. A tight-fitting filler 2 of stoic metal or other metal having a positive temperature coefficient, in an outer tube 1 of material, like nickel, having a negative temperature coefficient, will yield a vibrator the frequency of the vibrations of which is practically independent of temperature.

The two materials are united along their adjoining surface 3 in any desired way', as by welding, soldering, casting or pressing` the body 2 inside of the tube 1. The filler 2 may extend throughout the whole length of the tube 1, or it may be in the form of plugs 95, inserted in each end of the tube 94, with a hollow space 96 left near the center of the tube 1, as shown in Fig. 3.

The two dissimilar materials 1 and 2 may be in the form of concentric tubular shells '5 and 7, united, along their adjoining surface 3, as shown in Fig. 2. The shells may form a closed cylindrical body or, as illustrated, they may be left with a lengthwise gap at 9, to reduce hysteresis and eddy currents. The result is a longitudinally slotted magnetostrictive tube.

A Very efficient vibrator having a substantially zero temperature coeiicient is illustrated in Fig. 3, comprising a nickel tube 94 within which is driven a nickel-steel body 95 having about 36% of nickel. The body 95 used extended throughout the length of the tube 94. The outside diameter of the nickel tube used was 0.97 cm. and the tube wall was 0.088 cm. thick. The temperature coefficient was 1/65,000'per degree centigrade.

The body 95 may be replaced by lead, type metal or other alloys to reduce the frequency, and it may extend throughout the length of the tube 94, or, as shown at 96, the intermediate portion of the tube may be hollow. Thevibrator may have weights 97 and 98 attached to the ends thereof, which may be adjustable, if desired. In Fig. 3, the weights 97 and 98 take the form of split collars, which may be clamped in adjusted position by screws or the like 99. These expedients effectively decrease the period of vibration to comparatively low adjustable values.

It will be understood that, in all cases, at least one of the materials of the composite Vibrator is magnetostrictive, and the other material has a compensating property which causes a shifting of the Vcharacteristics of the vibrations toward the desired result.

Thus, as has been pointed out above, the compensation may consist in a modified frequency ofthe body. This is illustrated by the following example. The frequency of vibration of a thin nickel-tube vibrator about one meter long is 2,500 cyclesper second. A vibrator consisting of a nickel tube alone that would give 1,000 cycles per second would have to be 21/2 meters long, which is inconveniently large. All that is necessary, in order to obtain a vibration frequency of 1,000 cycles per second with a convenient, onemeter tube, is to fill it.

A third compensatory action, which is highly useful in frequency-measuring or frequency-controlling devices, is the elimination-of the effect of temperature on the frequency of vibration of magnetostrictive vibrators. This is attained, according to the present invention, as before explained, by combining a member having a positive temperature eoeiicient of frequency with a compensating member having a negative coeiiicient. The physical union of the two materials appears to have little, if any, effect upon their magnetostrictive properties. For the positive member, I prefer to use a nickelsteel alloy, such as has been referred to above, particularly a nickel-steel alloy of about the composition of invar. This is because of its availability and durability, and because it may be readily welded to the other material. Invar has the further advantage of being a strongly magnetostrictive body. I associate with the invar a compensatory body having a negative coefficient of frequency. For this I use preferably brass or some non-magnetic material, but I may also ciiicctively use Monel metal or nickel, or almost any other material of suitable elasticqualities, provided it has a negative temperature coefficient of frequency. The `relative thickness of the two bodies may be 'readily so chosen as to effect almost complete compensation for the effects of temperature on frequency of the composite system. I have derived theoretically the following formula for the frequency f ofvibration of /2 l- 2 Il where w and w are the respective weights of the two bodies, and f and ff are the respective frequencies of the two bodies, it being understood thatthe two bodies are of l.. the same length. This theoretical formula 'has proved to be-quite accurate, in practice.

As a deduction from this equation, by a process of differentiation not here given, I have derived the approximate result that if a and a are the increases of frequency of two vibrators, respectively, for a given small change of temperature, then the condition for a zero temperature coefiicient of the `composite system is that allujlfl: awf/ so that, when the frequencies and temperature coeflicients -of frequency of the two materials are given, Equation (2) permits a ready calculation. of the weights of the two materials which, associated together; will give a zero temperature coeflicient of frequency.

As an example, I- find that an invar strip increases in frequency 2.2 cycles in 10,00() per degree centigrade change in temperature; while a composite bar of invar with an equal longitudinal strip thickness of` brass welded to it has this coefficient reduced to 0.44 cycles in 10,000 per degree centigrade. The brass has a negative coeilicient which particularly compensates for the positive coefficient of invar. The particular thickness employed in this experiment was not suli cient to completely compensate, butl by using a piece of brass about 1.3 times the thickness of the invar, the compensation may be made complete. In practice, where the calculations, on account of insuliicient knowledge o f the separate materials, do not give an exactly zero temperature coeilicient of frequency, it is an easy matter, by trial and by filing away one of the materials,- to adjust the system to any required accuracy.

It is readily understood from the above disclosures that a large variety of materials, when given proper relative dimensions, may be used to replace the brass, and to some extent also there is liberty of choice among a limited range of known materials with positive temperature coefficients to replace the invar. and form a combination of negligible temperature coefficients of frequency. Because of this liberty of choice of materials it is preferred to describe the methods of temperature compensation in terms of the results obtained and the guiding formulae (l) and (2), which indicate to those skilled in the art, the proper preliminary design of one form of the novel vibrator,-such design to Lacasse and subsequent adjustment if necessary.

To persons skilled inthe art many other applications and modifications of the a paratus will occur, and no effort has here een made to be exhaustive.

What is claimed is: i

1. A composite magnetosqtrictive vibrator constituted of mechanicallyconnected members of different magnetostrictive material, one Aof the members being constituted of an outer shell in which other'portions of the vibrator are disposed.

2. A composite magnetostrictive vibrator constituted of mechanically connected members having different magnetostrictive properties, one of the members being constituted of an outer shell in which other portions of the vibrator are disposed, the said other portions of the vibrator beingV of different mechanical propertiesf such. as lead, type metal or various alloys.

3. A composite magnetostrictive vibrator constituted of mechanically connected members of different magnetostrictive material, 011e of the members being constituted of an outer shell in which other portions of the vibrator are disposed, a portion of the interior ofthe shell being hollow.

4f.- A composite magnetostrictive vibrator constituted of an outer nickel shell in which is mechanically secured a nickel-steel filler.

In testimony whereof, I have hereunto subscribed mv name.

GEORGE w. PIERCE.

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